Electroplating a ductile zinc-nickel alloy onto strip steel

ABSTRACT

A METHOD OF ELECTROPLATING A NICKEL-ZINC ALLOY ON A STEEL STRIP IS DESCRIBED, IN WHICH THE PLATING BATH IS MAINTAINED WITHIN A SELECTED PH RANGE, THROUGH USE OF A PRESELECTED RATIO OF NICKEL TO ZINC IN THE BATH, AND A PRESELECTED PLATING CURRENT DENSITY, WHEREBY AN ELECTROPLATED COATING IS FORMED HAVING A NICKEL CONTENT OF FROM 9.5 TO 12.5%, WHICH COATING HAS A LOW INTERNAL STRESS, AND INCREASED RESISTANCE TO CORROSION.

US. Cl. 20428 3 Claims ABSTRACT OF THE DISCLOSURE A method ofelectroplating a nickel-zinc alloy on a steel strip is described, inwhich the plating bath is maintained within a selected pH range, throughuse of a preselected ratio of nickel to zinc in the bath, and apreselected plating current density, whereby an electroplated coating isformed having a nickel content of from 9.5 to 12.5%, which coating has alow internal stress, and increased resistance to corrosion.

In Roehl Pat. No. 3,420,754, there is described a method ofelectroplating a ductile zinc-nickel alloy onto steel strip, in whichthe strip is caused to traverse an aqueous "United States Patent3,558,442 Patented Jan. 26, 1971 increasing the pH of the plating bath,and making certain changes in the ratio of the nickel to the Zinc in thebath, that the percentage of nickel in the electrodeposited coating maybe increased to a range of 9.5 to 12.5 while effecting a substantiallowering of the internal stress of the deposit, as well as a substantialincrease in the corrosion resistance of the coating.

In particular, we have found that in increasing the pH of the platingbath from 3.5 to 4.0, while increasing the nickel-zinc metal ratio inthe bath only slightly, i.e., from 0.56:1 to 0.60:1, the internal ordeposit stress is reduced from a contractile stress of +2700 p.s.i. top.s.i., and that by increasing the pH of the plating bath to 4.5, whileincreasing the nickel-zinc metal ratio in the bath to 0.63:1, theinternal or deposit stress is further reduced to an expansive stress of1200 p.s.i.

At the same time, the corrosion resistance or salt spray life, in hours,of the flat coating is increased from 122 hours to 160 hours and 156hours respectively, the salt spray life of the bent edge of the coating(at a bend of is increased from 133 hours to 156 hours and 172 hoursrespectively, and the salt spray life of the bent edge of the coating(at a bend of 180) is increased from 98 hours to 133 hours and 145 hoursrespectively.

The results of the foregoing and other tests are tabulated below forcomparison purposes:

NICKEL-ZINC ALLOY COATINGS ON STEEL STRIP pH of bath 4. 5 4. 0 3. 5 3. 02. 5

Average deposit stress, p.s.i -1, 200 +30 +2, 700 +3, 200 +4, 500 Saltspray life, hours:

Flat c0ating..- 156 160 122 145 Coating bent 90 172 156 133 133 122Coating bent 180 145 133 98 110 98 Bath composition:

Ni metal, oz.lgal 4. 9 5. 1 4. 5 4. 6 4. 4

Zn metal, oz./gal 7. 8 8. 5 8. 0 7. 8 7. 6

Ni-Zn metal ratio 0. 63 0. 60 0. 56 0. 59 0. 58

Acetic acid, percent of bath volume 2. 5 2. O 2. 0 2. 7 2. 4

Wetting agent, percent of bath volume 0. 2O 0. 19 0. 2O 0. 20 0. 20Operating conditions:

Bath temperature, F 130 130 130 130 130 Cathode current density, ampsper sq.

ft. of cathode surface 5O 50 50 50 50 Coating thicknes inches. 0001000012 00011 00011 00011 Percent nickel in coating 10. 3 9. 9 10. 7 10. 09. 9

plating bath having a pH of from 2.5 to 3.5, in which nickel chlorideand zinc chloride have been dissolved in sufficient amounts for eachgallon of the bath to have a zinc content of from 8 to 10 ounces and anickel content of from 4 to 5 ounces, and in which the strip is made acathode as it passes through the bath, and an electroplating currentdensity of 50 to 100 amperes per square foot of cathode surface of thestrip is maintained, whereby a zinc-nickel alloy coating iselectrodeposited on the steel strip, the alloy consisting of from 6.5 to9.5% nickel with the remainder zinc. The bath is maintained at atemperature of from to F.

The aforesaid coating is characterized by the fact that it has anexpansive internal stress of about 500 to 1400 p.s.i., and hassubstantially greater corrosion resistance than zinc.

In the interest of increasing the resistance of such coatings tocorrosion, it is desirable to increase the nickel content of the coatingto a range above 9.5%, but reference to the graph of FIG. 1 of theaforesaid Roehl patent indicates that the contractile internal stress ofthe coating rises very rapidly with such increase in nickel content, andsince such rise in contractile internal stress is a measure ofdecreasing ductility of the coating, ways and means have been sought byus to counteract this decrease in ductility.

We have discovered, through a series of tests, that by In the foregoingtests, the zinc in the bath is provided in the form of zinc chloride(ZnCl and the nickel in the form of nickel chloride (NiCl 6H O) Thewetting agent is used to lower the surface tension of the bath toeliminate pitting, if encountered, and to improve the uniformity ofdeposit appearance. The identity of the wetting agents chosen is notimportant, although they must, of course, be compatible with the bathand cause no deleterious effects. Some of the wetting agents used incommercial nickel plating have been found to be satisfactory, as, forexample, sulfated or sulfonated lauryl alcohol, one of the commonest soused. Others which produce less foam may be preferred, but the selectionof such Wetting agents most suitable for specific conditions is wellwithin the knowledge of those skilled in the art of electroplating,there being a wide variety available from purveyors of materials fornickel plating processes.

The subjects of internal residual stress, expansive internal stress andcontractile internal stress, and deposit stress are discussed in theaforesaid Roehl patent, to which reference may be made for anunderstanding thereof.

In general, we find that in our method of electroplating steel strip,the pH value of the bath should be maintained between 4.0 and 4.5, thebath temperature at 125-135 F., the current density at 40-100 a.s.f.,and the percentage of nickel in the deposit within the range of 9.5 to12.5%, with 11% as the optimum. Chanpes in the nickel content of thedeposit, within this range are efiected primarily by changing thenickel-zinc metal ratio in the bath. An increase in the pH value of thebath beyond 4.5 is undesirable, due to the fact that this results in anincrease in the expansive stress produced in the deposit.

While, in theory, maintenance of a zero stress is most desirable, it isto be noted that although an increase in the pH value of the bath to 4.5increases the expansive stress of the deposit to 1200, this iscompensated for by the resulting increase in salt spray life of the bentcoatings.

The following example illustrates a preferred embodiment of theinvention:

Having thus described our invention, we claim:

1. The method of plating steel strip with a zinc-nickel alloy whichcomprises: causing the strip to traverse an aqueous plating bath havinga pH of from 4.0 to 4.5, in which nickel chloride and zinc chloride havebeen dissolved in sufficient amounts for each gallon of the bath to havea zinc content of from 7 to 9 ounces and a nickel content of from 4 toounces; making said strip a cathode as it passes through said bath andmaintaining an electroplating current density of from 40 to 100 amperesper square foot of cathode surface of the strip, whereby a zinc-nickelalloy coating is electrodeposited on the steel strip, said zinc-nickelalloy consisting of from 9.5 to

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4 12.5% nickel with the remainder zinc, said coating having an internalstress of from to 1200, and which coating also has substantially greatercorrosion resistance than a coating produced under like conditions, buthaving a pH value of the bath of less than 4.

2. The method, as defined in claim 1, in which the plating bath ismaintained at a temperature of from 125 to 135 F.

3. A method of plating a metallic article with a zincnickel alloy whichcomprises: coating the article in an aqueous plating bath having a pH offrom 4.0 to 4.5, in which nickel chloride and zinc chloride have beendissolved in sufiicient amounts for each gallon of the bath to have azinc content of from 7 to 9 ounces and a nickel content of from 4 to 5ounces; making the article a cathode in the bath and maintaining anelectroplating current density of from to amperes per square foot ofcathode surface of the article, whereby a zinc-nickel alloy coating iselectrodeposited on the article, said zincnickel alloy consisting offrom 9.5 to 12.5% nickel with the remainder zinc, and which coating hassubstantially greater corrosion resistance than a coating produced underlike condition, but having a pH value of the bath of less' than 4.

References Cited UNITED STATES PATENTS 2,419,231 4/1947 Schantz 20443X2,840,517 6/1958 Faust et a1. 20443 2,844,530 7/1958 Wesley et a1.20443X 2,989,446 6/1961 Hammond et al. 204-43X 3,064,337 11/1962 Hammondet a1 29-196.5 3,420,754 1/1969 Roehl 20443 GERALD L. KAPLAN, PrimaryExaminer US. Cl. X.R. 204-43

